Add multi http definition

[rdubrock]

• Pull all the copy-pasted code from the k6 check into a shared utility
• Need to add accounting data -> I need help from someone on the team running Linux to make this happen, I think
• Validations are incomplete
• Tests
• Enforce a total test duration
• Handle k6 exiting with signal:killed

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[mem]

All these changes have been incorporated to the code. Thanks @rdubrock !